The present invention relates to anchoring bolts and, more particularly, to an expansion anchor for insertion into a cylindrical hole of a mounting substrate.
The expansion anchor includes an anchor bolt which, on a mounting end features threading and on an insertion end an anchoring part with at least one expansion cone flaring toward the insertion end. Extending from the cone end with the smaller diameter is a cylindrical neck section defining a bearing shoulder and, on the cone end with the larger diameter is a cylindrical coating zone. Coordinated with the expansion cone is an expansion element having the shape of a sleeve type expansion ring. The expansion ring is axially slotted and features several radial projections protruding beyond the outer shell of the expansion ring and distributed along the expansion ring circumference. The inside diameter of the projections is smaller than the largest cone diameter and larger than the neck section diameter, while the outside diameter of the projections at least equals the largest diameter of the anchor bolt.
An expansion anchor of that general type is known from DE-OS 28 28 983 and allows an especially fast setting of the anchor by simply driving it into a bore hole. The anchor is subsequently fixed in the bore hole by installing a nut on the external threading of the anchor bolt extending through the object to be mounted and tightening it. This causes the anchor bolt with its expansion cones to be moved in an axial direction, the expansion rings being expanded against the wall of the hole. The radially deformed expansion rings force themselves into the mounting substrate, for instance concrete, solid brick or lime sandstone forming a depression in it. To expand the expansion ring, a force must be applied via the expansion cones. For that purpose it is necessary that the expansion ring has sufficient friction with the mounting substrate. If this is not the case during a setting operation, axial slip and imprecise setting of the anchor will result. With the expansion anchor set, if the expansion ring does not have sufficient friction with the mounting substrate the occurrence of a crack in traction zones susceptible to cracking leads to an insufficient subsequent expansion. For anchoring the expansion ring of the prior art expansion anchor, several short triangular projections are provided on the outer shell of the expansion ring, which are arranged in an axial direction in the center area of the expansion ring, tapering toward its front end. This taper results in a disuniform contact pressure and thus an unfavorable digging which, in setting the anchor, frequently causes the expansion rings of the known expansion anchor to first be subject to slipping in the axial direction, so that a fixing of the expansion ring, as the installed nut is turned, will not immediately take place, and not at the original depth and position in the hole.
Known from DD 249 072 A1 is another expansion dowel featuring spike type triangular projections provided radially outside on the expansion ring, so that these projections will prevent a concomitant rotation of the expansion ring as the anchor is tightened, thereby guaranteeing a precise setting of the dowel. But it has been demonstrated that a digging of the spike type projections into the mounting substrate is not always sufficient, especially when the hole does not have an exact diameter corresponding to the anchor size, or if irregularities are present in the hole wall.
Furthermore, owing to the serrated design of the front inner edge of the expansion ring, in the insertion direction, there exists at the start of the expansion process a risk that the points of the spikes bite into the cone, leading to a faulty and incomplete expansion unnoticed by the user. A safe and uniform anchoring is thus not given.
Another expansion anchor is known from DE-OS 27 18 147. This patent disclosure teaches an expansion anchor which is expandable on both sides and is expanded on the front end, in the insertion direction, by a cone and on the rear end by a beveled transition between the relieved neck section on which the expansion shaft is arranged and the shaft of the bolt. The expansion ring features along its circumference, axial punch-outs which are engaged by noses provided on the cone. The punch-outs reduce the peripheral surface and thus the expansion surface of the sleeve against the hole wall. Since the load capacity of an anchor, until reaching maximum load, depends upon the size of the area which, prior to the start of expansion and up to the end of the expansion process is forced on the mounting substrate, a uniform force distribution of the tractive load is not given as compared to the expansion force, due to this loss of area.
Furthermore, there is a risk that the expansion ring may unintentionally seize on the bevel as the anchor is driven in, thereby preventing a deeper penetration of the anchor in the mounting substrate.
The forced expansion of the expansion ring occurring with this device in setting the anchor causes the latter to seize on the beveled transition between shaft and neck section. Thus, on account of the resulting friction losses, a considerably greater force must be expended for the proper expansion of the sleeve as the anchor is tightened.
The expansion dowel known from DE 25 54 851 C2 features a number of axial punch-outs on the front end, relative the insertion direction, so as to keep the force necessary for expansion to a minimum, due to the relatively easy bending of the tongues separated by the punch-outs. The large number of punch-outs leads to considerable losses of area and, thus, to a disuniform introduction of the tractive load in the hole wall and to an insufficient anchoring.
DE-OS 22 20 313 teaches an anchor bolt arrangement featuring an expansion sleeve with wart type beads arranged near the rear end, relative the insertion direction, in order to thereby guarantee a stationary tightening of the anchor. To facilitate the expansion of the expansion sleeve, a second gash extending beyond the center of the sleeve is provided opposite the longitudinal gash--viewed from the front end relative the insertion direction--which second gash extends into a round opening. The far to the rear location of the pivotal point of the expansion shells proves disadvantageous as the shells can be forced into the mounting substrate only at a relatively small angle to the bolt axis, achieving a relatively low burr effect in the mounting substrate.
DE-OS 27 20 939 proposes a self-expanding dowel. The use of this device though is limited to soft materials, for example building materials of poor quality, since the blocking element provided at the bottom end forms an expansion limit in hard mounting substrates, thus preventing a complete expansion of the sleeve.
To facilitate the initial bend-out of an expansion sleeve as a cone is drawn in, DE-GM 71 00 768 provides for an expansion sleeve which features an annular groove in the immediate vicinity of the front end relative the insertion direction. Provided for rotary locking in tightening the anchor are meshing axial grooves disposed on the inside sleeve and on the cylindrical bolt section bordering on it. A second groove corresponding to the first one is fitted on the opposite end of the sleeve. With this sleeve design it has proved to be disadvantageous in that a spring effect of the sleeve caused by the grooves is responsible for the fact that the resulting wrap friction will not sufficiently guarantee the expansion and easy final expansion of the sleeve.
Based on the discussed prior art, the problem underlying the present invention is to provide an expansion anchor of the initially named type which allows a safe setting without slip, allows the application of high traction forces and a durable, safe fastening in traction zones susceptible to cracking.